Protecting against corrosion by passification

Transcription

1 Teacher's/Lecturer's Sheet Protecting against corrosion by passification (Item No.: P ) Curricular Relevance Area of Expertise: Chemistry Education Level: Age Topic: Physical chemistry Subtopic: Electrochmistry, potentials, conductivity and electrolysis Experiment: Protecting against corrosion by passification Difficulty Preparation Time Execution Time Recommended Group Size Intermediate 10 Minutes 10 Minutes 2 Students Additional Requirements: Experiment Variations: Keywords: corrosion, passification, corrosion protection Information for teachers Introduction Some base metals, for example aluminium, become relatively resistant to corrosion by formation of a dense surface film of oxide. However, the protective oxide films are mostly very thin, so that they can only resist very aggressive chemicals for a limited time. The stability of such metals can be increased by an artificial strengthening of the oxide film, a process known as passification. Two different procedures are presently used in practice. These are: 1. Passification of the metal by a brief action of concentrated nitric acid on its surface, and 2. Anodic oxidation of the metal, carried out by hanging it as anode in an electrolysis bath containing dilute sulphuric acid, whereby the nascent oxygen which is evolved strengthens the oxide film. The Eloxal procedure for treating the surface of aluminium has made this procedure particularly well known. Educational objective The students passify iron with nitric acid and subject aluminium to anodic oxidation. Equipment Preparation: Production of the copper sulfate solution For the production of the 0.2 M copper sulfate solution 7.98 g pure copper sulfate or g copper sulfate-5-hydrate are weighed out and destilled water is added up to a volume of 250 ml. Solve the copper sulfate by stirring.

2 Teacher's/Lecturer's Sheet Safety information H and P statements Copper sulfate sulution H410 P273 P501 Very toxic to aquatic life with long lasting effects. Avoid release to the environment. Dispose of contents/container to admitted waste disposal systems. Sulfuric acid 2% H290 H314 P280 May be corrosive to metals. Causes severe skin burns and eye damage. Wear protective gloves/protective clothing/eye protection/face protection. Nitric acid 65% H272 H290 H314 P260 P280 P301 + P330 + P331 P305 + P351 + P338 P309 + P310 P501 May intensify fire; oxidiser. May be corrosive to metals. Causes severe skin burns and eye damage. Do not breathe dust/fume/gas/mist/vapours/spray. Wear protective gloves/protective clothing/eye protection/face protection. IF SWALLOWED: rinse mouth. Do NOT induce vomiting. IF IN EYES: Rinse cautiously with water for several minutes. Remove contact lenses, if present and easy to do. Continue rinsing. IF exposed: Call a POISON CENTER or doctor/physician. Dispose of contents/container to admitted waste disposal systems. Hazards Disposal Concentrated nitric acid is highly caustic and vapours with a biting smell rise up from the solution. Nitric acid solution of concentration c = 0,5 mol/l acts as irritant. Protect eyes and skin. Completely avoid contact of the chemicals with eyes and skin. Do not breathe spray! Wear protective gloves and protective glasses! After neutralisation (ph 6 to 8) pour acids and bases into the sink, heavy metals and solutions containing heavy metals are given into the canister for heavy metal waste.

3 Protecting against corrosion by passification (Item No.: P ) Introduction Application and task Application Some base metals, for example aluminium, become relatively resistant to corrosion by formation of a dense surface film of oxide. However, the protective oxide films are mostly very thin, so that they can only resist very aggressive chemicals for a limited time. The stability of such metals can be increased by an artificial strengthening of the oxide film, a process known as passification. Two different procedures are presently used in practice. These are: 1. Passification of the metal by a brief action of concentrated nitric acid on its surface, and 2. Anodic oxidation of the metal, carried out by hanging it as anode in an electrolysis bath containing dilute sulphuric acid, whereby the nascent oxygen which is evolved strengthens the oxide film. The Eloxal procedure for treating the surface of aluminium has made this procedure particularly well known. Task A passification of iron with nitric acid and of aluminium by anodic oxidation shall be done. Equipment Preparation: Production of the copper sulfate solution For the production of the 0.2 M copper sulfate solution 7.98 g pure copper sulfate or g copper sulfate-5-hydrate are weighed out and destilled water is added up to a volume of 250 ml. Solve the copper sulfate by stirring. Set-up and procedure Hazard Hazards Concentrated nitric acid is highly caustic, spray is pungent. Nitric acid solution of concentration c = 0,5 mol/l acts as irritant. Protect eyes and skin. Completely avoid contact of the chemicals with eyes and skin. Do not breathe spray! Wear protective gloves and protective glasses!

4 Passification by nitric acid Set-up Place a beaker containing a little concentrated (65%) nitric acid in the measuring cell block. Fill the beaker at most up to a height of 3 cm! Take care when handling concentrated nitric acid! It is one of the most aggressive acids! Place a beaker containing dilute copper sulphate solution (approx. 0.2 molar, fill this beaker also at most up to a height of 3 cm) next to the first beaker (see picture 1). Procedure Picture 1: Set-up of the passification by nitric acid Thoroughly clean the surface of one end of a strip of iron by rubbing it down with fine sandpaper. It must be completely free of residues of grease or oil, or of any other contaminant. After sandpapering it, just wipe the cleaned surface with a dry paper towel or cotton wool. Dip the cleaned end of the metal strip as deep as possible into the concentrated nitric acid for about 2 seconds and then into the copper sulphate solution for about 2 seconds. Subsequently keep watching the immersed end of the metal strip. Note: Should this coating not appear within about 15 seconds, briefly hit on the immersed area with a hard object (glass rod, nail etc.). The copper plating then usually takes place at once, starting at the point which was hit. Passification by anodic oxidation Set-up Fill two neightbouring cells of the measuring cell block with dilute, approx. 2% sulphuric acid, and conductively connect them with a strip of filter paper which is also wetted with this dilute sulphuric acid. Place a cover on one cell and insert a carbon electrode as cathode in this. Place a strip of aluminium (about 60 mm long) in the other cell as anode as shown in picture 2. Connect up a circuit consisting of a source of direct voltage (transformer with rectifier) with 12 to 15 volts, a measuring instrument (set at 200 ma) and the two electrodes (see picture 3).

5 Picture 2: Set-up of the electrodes Procedure Picture 3: Set-up of the passification by anodic oxidation Switch on the current and carry out electrolysis for about 10 minutes. Then take the aluminium strip out of the sulphuric acid, rinse it and dry it. Experiment for ecidence Set the source of direct voltage to about 2 V. Connect up a circuit from the source of current over the electric motor (with the disk fitted on) to the aluminium sheet. One connection to the aluminium is made by a crocodile clip firmly fitted to the nonanodized section of it. The second connection is made with a plug which is used as feeler on the anodized and non-anodized sections of the aluminium.